The experiments outlined in this proposal focus on RNA-RNA interactions in Flock House virus (FHV), a nonenveloped, icosahedral insect virus. The genome of FHV is split between two distinct, single-stranded, messenger-sense RNA molecules which are thought to be encapsidated into a single virion. The mechanism by which one copy of each genomic RNA is selected for packaging into a single particle is unknown. Preliminary studies from our laboratory have demonstrated that mild heating of purified virions induces formation of a stable heterodimeric RNA complex within the particle suggesting that the encapsidated nucleic acids undergo significant structural rearrangements. Furthermore, mild dissociation of FHV provirions has revealed that this complex may already exist prior to heating, albeit in a more labile form. Heterodimer formation is hypothesized to provide a means by which the two RNAs are copackaged during assembly of virus particles. In this application, we intend to isolate the labile heterodimer from purified virions and/or reconstitute it in vitro and plan to characterize it using various biochemical, physical and molecular genetic methods. In addition, in collaboration with Dr. John E. Johnson's laboratory, a newly developed low resolution X-ray crystallography technique will be utilized to visualize changes in RNA structure induced upon heating of FHV particles. The results from the proposed studies should provide new insights into the mechanisms of RNA packaging in segmented viruses and the organization of the nucleic acid inside the virion. This knowledge could provide a basis for developing strategies of how to interfere with the life cycle of medically important viruses.